1. Field of the Invention
This invention relates to improvements in a composite paint film coated on a steel plate to improve rust resistance of the steel plate and a gloss and brightness character of the coated paint film coating, and more particularly to such an improved composite paint film to be coated on the outer panel of an automotive vehicle.
2. Description of the Prior Art
In general, a composite paint film for an automotive vehicle outer panel is constituted of an electrodeposition paint film for the purpose of rust prevention, an intermediate coat paint film for the purpose of thickening the composite film, and a finish coat paint film for the purpose of coloring. The automotive vehicle outer panel coated with such a composite paint film is required to have higher smoothness, gloss and the like. Total evaluation of such characteristics has been usually accomplished with a so-called PGD value which is measured by a PGD (Portable Gloss and Distinction) meter. This PGD meter is, for example, a PGD-3 type one produced by the Japan Colour Research Institute in Japan. It is known in the skilled in one art, that the PGD value becomes higher as the smoothness and gloss of the finish coating film is higher. Here, the gloss (or a gloss and brightness character) of the composite paint film coating depends on the performance of the finish coat paint film and seems to be an independent function of a finish coat paint. Concerning the smoothness, it is largely affected by the smoothness of under-coat films and therefore the electrodeposition paint film is eagerly required to have a high smoothness.
In conventional electrodeposition films, a measure to raise fluidity (i.e., to lower its melt viscosity) during baking has been taken in order to improve the smoothness of the electrodeposition film. There is an Oscillated Pendulum Analysis (OPA) with an oscillated pendulum type viscoelasticity measuring device, as a method of measuring the fluidity of an electrodeposition film during baking. The degree of the fluidity can be represented with a minimum melt viscosity (.lambda. min) determined by the measuring method.
Conventional electrodeposition paints have a minimum melt viscosity (.lambda. min) of 0.15 or lower according to the above measuring method. In connection with such electrodeposition paints, intermediate coat paints are designed to harden for as short a time as possible in order to ensure the smoothness of the coated paint film on a horizontal plane and to prevent deterioration of the smoothness of the same film on a vertical plane on which smoothness deterioration is caused by run of the coated paint film during baking. Additionally conventional intermediate coat paints have a hardening starting time, measured by the Oscillated Pendulum Analysis (OPA), from 2 to 15 minutes. Thus, the conventional composite paint films are expressed to be formed by coating the intermediate coat paint film having a hardening starting time shorter than 15 minutes (according to OPA) on the electrodeposition paint film having a minimum melt viscosity (.lambda. min) of 0.15 or lower (according to OPA).
In such conventional paint films, measures to increase the flowability of the coated electrodeposition paint film as discussed above have been taken. However, under the action of surface tension of the molten coated film at an edge portion of the steel plate of the automotive vehicle outer panel, the edge of the steel plate is exposed without the coat film, so that the above-discussed conventional composite paint film is inferior in so-called edge corrosion resistance.
If the edge corrosion resistance is intended to be improved in the conventional composite paint films, it may be proposed to increase-the viscosity of the electrodeposition paint film while in a molten condition. For this purpose, measures have been proposed to increase the concentration of pigment in the electrodeposition paint (i.e., P(pigment)/B(binder) ratio), a measure to add non-molten type cross linking resin particles (i.e., a reology control agent) to the electrodeposition paint, and a measure to lower the hardening temperature to suppress flowability of the electrodeposition paint in a molten state under a cross-linking reaction. These measures are disclosed, for example, in U.S. Pat. Nos. 3984299 and 4017438, and Japanese Patent Provisional Publication No. 59 43013. However, any of these measures largely degrades the flowability of the electrodeposition paint in a molten state. Assuming that these measures are taken to obtain a good edge corrosion resistance, the smoothness of the coated surface of the electrodeposition paint is unavoidably deteriorated. Accordingly, in a case in which conventional intermediate and finish coats are formed on the thus formed electrodeposition paint film, the PGD value of the finish coat paint film on a horizontal plane largely lowers, thereby degrading the commercial value of the resultant automotive vehicle.
In view of the above, it has been eagerly desired in an automotive industry to obtain a composite paint film exhibiting both high edge corrosion resistance and good external appearance.